An electronic timepiece (especially wristwatch) with solar cell, in which electric power generated by a solar cell is accumulated in an electric power accumulation device (referred to as "accumulator device" hereinafter), and time keep and time display are performed with the accumulated electric power, has recently become widespread.
From the above it has been supposed that the convenience of freedom from trouble of battery replacement and the recent requirements to prevent environmental pollution due to battery waste, are supported by users of wristwatches.
The most basic configuration of a conventional electronic timepiece with solar cell is shown in FIG. 8.
In this electronic timepiece with solar cell, electric power generated by a solar cell 101 is accumulated in an accumulator device 104 and a time keeping means 105 is driven by the accumulated electric power. As the solar cell 101, a plurality of cells (generally about four cells) made by vapor deposition of an amorphous silicon thin film onto a substrate and connected in series, are widely used. As the accumulator device 104, a secondary cell (battery) is used.
The time keeping means 105 is an electronic timepiece module comprising a time keep counter circuit having a quartz oscillator circuit, an electric counter circuit (frequency dividing circuit) and the like, a digital display or hands and a drive mechanism thereof for displaying a time and the like, and so on.
A reverse flow preventing diode 802 is provided to prevent the accumulated electric power from decreasing due to a reverse flow of an electric current from the accumulator device 104 to the solar cell 101 in a state where the generated voltage by the solar cell 101 is low due to weak ambient light.
However, even if the reverse flow preventing diode 802 can serve to prevent leakage current in a state where the generated voltage by the solar cell 101 is low in low intensity of illumination as described above, it conversely has the disadvantage when the intensity of illumination is high and therefore the generated voltage by the solar cell 101 is high, a forward voltage drop of about 0.5 V due to the reverse flow preventing diode 802 produces a loss, which causes the charging efficiency of the accumulator device 104 to lower.
The influence of the voltage drop of about 0.5 V due to the reverse flow preventing diode 802 becomes a more significant problem in an electronic timepiece using a solar cell in which the number of cells connected in series (the number of divided surface electrodes) is few.
This is because the generated voltage by one cell of the solar cell is about 0.5 V, and it is multiplied by the number of cells connected in series to be a value of the generated voltage of the solar cell, therefore a solar cell having a small number of cells connected in series has a low output voltage. Especially in a single cell having the number of cells connected in series =1, almost all of the generated voltage by the solar cell 101 is consumed in a forward voltage drop of the reverse flow preventing diode 802, thereby operational conditions for charging the accumulator device 104 are not established.
For the accumulator device in such an electronic timepiece with solar cell, an accumulation means having a large capacitance is used so as to drive its time keeping means for a long time even in a state where the solar cell does not generate electric power. Therefore, there is a disadvantage in that in a state where the accumulated electric power has been almost totally consumed, it takes a long time after illuminating the solar cell again before enough electric power is accumulated in the accumulator device to cause the time keeping means to start its operation.
To solve the above disadvantage, there exists a quick start type electronic timepiece having a configuration in which a large capacitance accumulator device and a small capacitance accumulator device are provided in parallel, and when light is emitted to a solar cell to start generating electric power in a state where the accumulated electric power in the accumulator device has been almost totally consumed as described above, the small accumulator device is first charged by the generated electric power and then the time keeping means can start its driving in a short time with the accumulated electric power.
An example of the configuration is shown in FIG. 9. In the quick-start type electronic timepiece with solar cell, a small capacitance accumulator device (capacitor) 905 and a large capacitance accumulator device (secondary cell) 906 are connected in parallel to the solar cell 101 via reverse flow preventing diodes 901 and 902 in place of the accumulator device 104 shown in FIG. 8.
The small capacitance accumulator device 905 is directly connected to the time keeping means 105 in parallel, and a switch 903 for selecting an object to be charged is inserted between the large capacitance accumulator device 906 and the diode 902, and a switch 904 for selecting a power source is also inserted between the large capacitance accumulator device 906 and the time keeping means 105.
Furthermore, a voltage detection means 907 is provided in the above timepiece to detect the value of the accumulated voltage Vb of the large capacitance accumulator device 906 and to electrically control the on-off states of the switches 903 and 904 in accordance with the detection result. More specifically, when the accumulated voltage Vb of the large capacitance accumulator device 906 is below a preset value, the control of bringing the switch 903 into the on-off states at a predetermined ratio is repeated, thereby the small capacitance accumulator device 905 is quickly charged and the large capacitance accumulator device 906 is gradually charged. At this time, the switch 904 is kept in the off-state, thereby the time keeping means 105 is instantly driven with the accumulated electric power in the small capacitance accumulator device 905.
In a state where the accumulated voltage Vb of the large capacitance accumulator device 906 exceeds the preset value, the switches 903 and 904 are both kept in the on-states and the large capacitance accumulator device 906 is selected both as an object to be charged and as the power source of the time keeping means 105.
Also in this electronic timepiece, there is a disadvantage that since reverse flow preventing diodes 901 and 902 are provided in the respective charging paths of the small capacitance accumulator device 905 and the large capacitance accumulator device 906 to prevent leakage currents via the solar cell 101 in a weak ambient light, voltage drops occur due to the reverse flow preventing diodes 901 and 902 when charging with the generated electric power by the solar cell 101, which causes the charging efficiency for the respective accumulator devices 905 and 906 to be lower.